In an engine which has means for heating the catalytic converter, be it an afterburner or an electrical heating element, a system is required for controlling the heating means in order to avoid possible damage to the catalytic converter through overheating. The control system should, for example, switch off the heating means once the catalytic converter has reached its light off temperature, prevent the heating from working during a warm start, and reactivate the heating means if the catalytic converter should drop below its light off temperature through prolonged low load operation of the engine.
Clearly, it is possible to make a direct measurement of the temperature of the catalytic converter. This of course is disadvantageous from the point of view of cost but a more serious objection to the use of a temperature sensor is the issue of reliability. Proposed legislation requires vehicles to comply with emission regulations not only when they are first manufactured but also after several tens of thousands of miles of use (currently 50,000 miles, 100,000 miles in the future) and the lifetime of a sensor operating in such a hostile environment cannot be guaranteed over such a period.